Synthetic fibers, particularly polyester fibers show poor color development when dyed, and various investigations have heretofore been carried out on this problem. For example, Japanese Unexamined Patent Publication (Kokai) No. 52-99400 and Japanese Examined Patent Publication (Kokoku) No. 60-37225 disclose an improvement in the deepness of the color of the fibers by imparting a fine unevenness on the surface of the fibers, by plasma etching, so that the absorption of light is increased. Although the method is effective in improving the deepness of black shade mainly caused by absorption of light, substantially no effect of improving the color developing property with respect to a chromatic color is observed.
On the other hand, a method of improving the degree of dyeing of polyester fibers by copolymerizing the polyester polymer constituting the polyester fibers with a third component has been widely known. For example, Japanese Unexamined Patent Publication (Kokai) No. 6-173114 discloses a polyethylene terephthalate yarn obtained by copolymerizing with an aliphatic dicarboxylic acid which has from 4 to 10 total carbon atoms and a glass transition temperature of up to 70.degree. C. The patent publication also discloses that when the yarn is dyed, the yarn shows a high degree of dyeing compared with a conventional polyethylene terephthalate yarn, that is, the dyed yarn shows a low L* expressed in the L*a*b* color specification system recommended by International Illumination Commission (Commission Internationale de l'Eclairage (CIE)) and specified by JIS Z8729-1980.
However, the L* is an index showing the brightness of color, namely, the "deepness of color" of a dyed yarn,. Accordingly, the method is intended to dye a yarn "deeply". In the intention, there is no recognition that the "chromaticness of color" represented by a C* in the L*a*b* color specification system of CIE, i.e., the brightness and clearness are improved.
Furthermore, as disclosed in the patent publication, when the polyethylene terephthalate yarn copolymerized with a third component such as an aliphatic dicarboxylic acid is dyed by a conventional method to merely increase the degree of dyeing (L* being decreased), there arises the problem that the color becomes dark and the brightness and clearness (C*) are rather decreased.